1. Field of the Invention
The present invention relates to a light emitting diode (LED) array employable in an electronic printer for photography, and particularly to an improvement of light emitting windows in the LED array.
2. Description of Prior Arts
FIG. 1 illustrates the structure of a conventional LED array 1. The LED array 1 comprises a semiconductor body 10 having an N-GaAs layer 3, an N-Al.sub.x Ga.sub.1-x As layer 4 and P-Al.sub.x Ga.sub.1-x As regions 5. An N-electrode layer 2 is formed on the bottom surface of the semiconductor body 10, while an insulating layer 8 is provided on the top surface of the semiconductor body 10. The insulating layer 8 has a linear array of rectangular windows 9, which are defined on the P-regions 5. P-electrode layers 11 are formed on the insulating layer 8 and respective end portions thereof are in contact with the top surfaces of the P-regions 5 through the windows 9.
FIG. 2 is a schematic diagram showing the situation in which the LED array is employed in an electronic printer for photography. The LED array 1 is fixed to a base plate 21 such as a printed board, and the electric connection between the LED array 1 and the base plate 21 is attained through die bonding and wire bonding. A self-focus lens array (SLA) 22 is supported by a supporting member (not shown) so that the windows 9 (FIG. 1) face to the SLA 22 across a gap therebetween. A photosensitive drum 23 is rotatably provided and the center axis of the drum 23 is located on a line passing through the LED array 1 and the SLA 22.
An image signal is transmitted to an LED drive circuit (not shown), and in response to the image signal, the LED drive circuit selectively supplies electric power to the LEDs included in the LED array 1 through the electrodes 2 and 11. The LEDs supplied with the electric power generate light L1 at the PN junctions thereof and emit the light L1 through the windows 9. The SLA 22 converts the light L1 into converging light L2, which is applied to the photosensitive surface 24 of the drum 23. The SLA 22 is effective for preventing the light spot from spreading on the photosensitive surface 24 of the drum 23.
Prior to the exposure to the light L2, the photosensitive surface 24 of the drum 23 is electrostatically charged to a negative level. When the light L2 is applied to the photosensitive surface 24, only the part of the surface 24 exposed to the light L2 is discharged. Therefore, a lattent image is obtained on the photosensitive surface 24 by repreating the light generating process while rotating the drum 23 around the axis thereof. Then, the photosensitive surface 24 is supplied with toner charged to a negative level, and the toner stays only on the discharged part of the surface 24 since the toner cannot stay on the non-discharged part due to electrostatic replusive force. The toner staying on the surface 24 is transferred onto a paper, and then, the image thus obtained is fixed by heart and pressure.
The transmission efficiency of light is about several percent in the LED array head consisting of the LED array 1, the base plate 21, the SLA 22 and a driver (not shown). This is because the SLA 22, which has a relatively low transmission efficiency of light, should be provided in the conventional LED array head. Therefore, it has been desired to provide an LED array which can be used at a high transmission efficiency of light.